1. Field of the Invention
This invention pertains to improvements in electronic circuitry used in moving read/write heads in a memory disk system of the type used in computer systems or the like, and, more particularly, to improvements in such circuitry for providing linear drive signals to a "voice coil" of such system.
2. Relevant Background
A coil that serves as an actuator to move and position the read/write heads of a memory disk drive of the type used in a computer system is widely referred to as a "voice coil". Such memory disk drives are typically found in so-called "hard-disk" drives, CD ROM drives, and so on. The actuator coils are known in the art as "voice coils" due to their similarity to coils commonly used in audio loud speaker systems or the like. Such voice coils are operated in a manner similar to acoustic voice coils since a positive current applied to the coil produces a corresponding positive direction of motion of the read/write head. A negative current applied to the voice coil produces a corresponding negative displacement of the read/write head.
Ideally, the system is linear, so that the acceleration of the read/write head is directly proportional to the magnitude of the current applied to the voice coil. However, in distinction to acoustic voice coils, the voice coils used in disk drive systems typically drive a read/write head mechanism that has virtually no suspension or spring mechanism. Thus, a dc current must be capable of being continuously provided to the voice coil to establish and maintain the desired position of the read/write head mechanism.
In operation of voice coil systems used in memory disk drives, two modes are generally provided. The first mode is referred to as a "seek" mode in which large currents are applied to the voice coil to rapidly bring the head mechanism and the read/write head to the point or track on the disk media at which the information desired to be read or written is located. Then, a "track-follow" (TF) mode is entered in which smaller currents are applied to the voice coil to maintain the position of the read/write head on the desired track of the disk media.
In the track-follow mode, tracking information commonly emplaced on the disk memory media is used in maintaining the head position with respect to the track through various known servomechanism techniques. It will therefore be appreciated that in the track-follow mode, a high degree of precision in the head placement is required to accurately read or write the data.
Because of the high degree of linearity that is required in moving and positioning the read/write head mechanism, and because of the relatively large currents which are required in the "seek" mode, typically the amplifiers employed in a voice coil driver system are operated as "Class AB" amplifiers.
One of the advantages of a Class AB amplifier stage is that at the crossover point, or change of direction of the drive current supplied to the voice coil, discontinuities are relatively small. This is due, in part, to the fact that there is always biasing current flowing through the output stages. Accordingly, generally one amplifier stage does not completely turn off before the other turns on in a current reversing event. As a result, the transfer function of the currents through the load is relatively linear through the origin as the current changes sign. The disadvantage, of course, is that the circuit has power dissipation, even when the load current is at a minimum or zero. This is not insignificant, especially in low power devices such as portable computers or the like.
An electrical schematic diagram of a typical voice coil driver system 10 of the prior art is shown in FIG. 1. The voice coil 11 shown may be a part of the read/write head mechanism of a disk drive of the type used in personal, portable, or other computers. Two amplifiers 13 and 14 are provided for delivering drive current to a voice coil 11.
An input voltage, "V.sub.IN ", is applied to the circuit 10 from a servocontroller with which the voice coil system 10 is associated. Ideally, the relationship between V.sub.IN and the current applied to the voice coil is directly proportional.
The amplifiers 13 and 14 are generally connected to serve as a bridge driver for the voice coil 11. It will be appreciated that the driver circuit 10 is a high performance device, and, therefore, requires precise components for implementation. Moreover, in order to achieve the precision needed, typically, a large number of operational amplifiers are employed, again, each with precision components. Additionally the amplifiers 13 and 14 of the system 10 operate in Class AB mode. Therefore, as mentioned, the biasing currents used within the Class AB amplifiers is always flowing.
In the past, efforts have been advanced to provide amplifiers in voice coil driver circuits that are not Class AB amplifiers, in order to enable drive currents of increased magnitude to be readily accomplished. However, such driver circuits typically have either "deadbands" or "jumps" in their transfer functions that are undesirably large. A "jump" in the transfer function results in a discontinuity as the input voltage changes sign, and results in the output current jumping from a negative value to a positive value discontinuously at zero volts input. A "deadband", on the other hand, is a discontinuity in which the output current remains at zero as the input voltage changes sign. Thus, in order to be enabled to use amplifiers other than those of Class AB type, the jump and/or deadband discontinuities must be minimized to enable the heads to be accurately positioned and maintained at the desired position.
The main advantage, of course, in enabling a non-Class AB type operation is the reduction or elimination of the biasing currents that are required in Class A or AB type operation.